The invention concerns the application of coverings and/or elements to the surface of rubber articles, in particular tires. More particularly, it concerns the application of these elements and/or coverings after the vulcanization of such articles. The invention concerns in particular the marking, colored or not, of part of the surface of the tire by applying a rigid or non-rigid element and/or a covering.
The tires can carry, for decorative or identification purposes, a marking, colored or not, directly applied to part of the generally external surface of the tire. “Marking” means, in the text, any element or covering affixed to the said surface of the tire, generally after its vulcanization. This marking can consist of a so-called rigid element, a soft element, a layer of lacquer, a coat of paint, etc.
“Rigid element” means elements incapable of following the successive deformations of a tire when it is running. These so-called rigid elements can according to circumstances be completely non-deformable or flexible but non-elastic to the scale of the deformations of the tire. In other words, they can, where necessary, bend but be incapable of following the local variations in the area of the surface of the tire following changes in radius of curvature of the said tire. These elements can consist of metal, plastic, elastomer, ceramic, etc.
The marking of a tire, produced in an appropriate material, can be in one or more colors, including white. However, producing markings, colored or not, on tires poses great difficulties.
Such markings are intended to cover at least part of the surface of tires. Since it is a case of a surface covering, these markings are particularly intended to cover surfaces of tire covers said to be “non-wearing”, that is to say which are not in contact with the road under normal conditions of use of the said tires. Thus such colored markings, having decorative and identification purposes in particular, can constitute at least partially the external surface of a tire cover sidewall or groove bottoms of the tread.
The complexity of directly covering the surface of tires with such markings is well known to persons skilled in the art. There exist in fact two major difficulties in this implementation, firstly the nature of the compositions in the tire which acts on the markings, as will be seen more explicitly below, and secondly the stresses to which the tire is subjected when it is running which cause significant deformations of the tire and therefore of its surface, to which the markings must be able to adapt.
The vulcanized compositions of diene rubbers, both natural and synthetic, because of the presence of double bonds on their molecular chains, are liable to deteriorate more or less rapidly after prolonged exposure to the atmosphere, because of known mechanisms of oxidation and ozonolysis. These degradation mechanisms are also accelerated under the combined action of heat by thermo-oxidation, or that of light by photo-oxidation (see for example: “Photo-oxydation and stabilisation of polymers”, Trends in Polym. Sci., Vol. 4, No 3, 1996, 92-98; “Degradation mechanisms of rubbers”, Int. Polym. Sci. and Technol., Vol 22, No 12, 1995, 47-57).
It has been possible to inhibit all these phenomena of degradation of the tire, linked to its aging, little by little by virtue of the development and introduction into diene rubber compositions intended for the manufacture of tires of various antidegrading agents such as antioxidants or antiozonants. Compounds have even been found which are able to fulfil these two functions simultaneously; the most effective, both as antioxidants or antiozonants, are in a known manner derivatives of quinoline, such as for example 2,2,4-trimethyl-1,2-dihydroquinoline (“TMQ”) or derivatives of p-phenylenediamine (“PPD”) even more active than the first, such as for example N-1,3-dimethylbutyl-N′-phenyl-p-phenylenediamine (“6-PPD”) or N-isopropyl-N′-phenyl-p-phenylenediamine (“I-PPD”). These antidegradants of the TMQ and PPD type, sometimes even combined, are today very widespread and used in an almost routine manner in conventional diene rubber compositions filled at least partly with carbon black, giving them their characteristic black color.
However, the agents cited above, in particular TMQ or PPD derivatives, are not stable under light and, under the effect of UV radiation, generated colored chemical substances which cause an impairment of the color of the rubber compositions containing them, staining them dark brown.
In addition, antidegradants are mobile on the molecular scale within the rubber composition containing them. This mobility extends to the compositions adjacent to that initially containing the antidegradants. Thus these antidegradants are liable to migrate into a colored marking composition fixed to the rubber composition containing them, and to stain it.
Moreover, in order to limit the degradation due to ozone, elastomer compositions also normally incorporate waxes which, statically, afford additional protection with respect to antiozonants by the formation of a protective surface coating. However, the waxes, which are also characterised by their ability to migrate as far as the surface of the rubber articles, modify the external appearance of the surfaces of the elastomer compositions by staining them or making them dull and grey. This phenomenon is referred to as the “blooming” of the waxes.
This is why it is extremely complex to produce a marking of durable appearance, colored or not, intended to cover part of the external surface of tire covers resisting staining and alteration due to the antioxidants, antiozonants and waxes present in the elastomer compositions used in tires.
The majority of solutions propose the development of color markings whose composition combines both the various constituents for obtaining color and non-migration of the waxes or staining products. However, it is also necessary, as stated, for the marking to be capable of enduring the mechanical stresses to which the tire is subjected.
However, the solutions to these two problems which are accessible at the present time are often opposing and are therefore not satisfactory for improving the compromise between durable appearance and resistance to mechanical stresses. This is because:                when the covering is sufficiently flexible to accommodate repeated deformations throughout the life of the tire, the molecular mobilities are too great therein for completely preventing the migrations of these staining products;        when the coating is rigid, thus restricting the molecular mobility, it is no longer capable of following the deformations of a tire without causing excessively great concentrations of stresses which, according to circumstances, will result in a detachment or breakage of the covering.        
It therefore proves necessary, in order to obtain durable marking, to be able at least to reduce or even eliminate the impairment of the color and/or the brightness of the marking affixed to the external surface of a tire, whilst conferring on it an ability to accommodate deformations of the tire.